1. Field of the Invention
The present invention relates to a method of making a keeper layer for a spin valve sensor with low intrinsic anisotropy field and more particularly to a method of making a keeper layer that is more reliable for counterbalancing demagnetizing fields and sense current fields on a free layer when a sense current is conducted through multiple layers of the spin valve sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk or a linearly moving magnetic tape. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree. to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative magnetic fields from a moving magnetic medium. The quiescent position is the position of the magnetic moment of the free layer with the sense current conducted through the sensor and without magnetic field incursions from a rotating magnetic disk. The quiescent position is preferably parallel to the ABS. If the quiescent position of the magnetic moment is not parallel to the ABS when there is no magnetic field incursion from the disk the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen to be less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. A spin valve sensor has a significantly higher magnetoresistive (MR) coefficient than an anisotropic magnetoresistive (AMR) sensor. For this reason it is sometimes referred to as a giant magnetoresistive (GMR) sensor. Typically, GMR sensors simultaneously display both AMR and GMR effects, so that the output signal is a superposition of their AMR and GMR signals.
The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos .theta.. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative magnetic fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the magnetoresistance (MR) of the spin valve read head be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS, and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no magnetic field signals from the magnetic disk) the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative magnetic fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric.
The location of the bias point on the transfer curve is influenced by five major forces on the free layer, namely a ferromagnetic coupling field (H.sub.FC) between the pinned layer and the free layer, a demag field (H.sub.demag) from the pinned layer, demagnetization (demag) field of the free layer acting on itself, sense current fields (H.sub.SC) from all conductive layers of the spin valve except the free layer, and the superposition of the AMR signal on the GMR signal. The influence of the AMR effect on the bias point is also defined in terms of magnitude and direction.
When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer in a first direction. In addition, the pinned layer demagnetization field further rotates it counteracted by a ferromagnetic coupling field H.sub.FC imposed by the pinned layer on the free layer. A ferromagnetic keeper layer is employed on an opposite side of the free layer with a nonmagnetic electrically conductive second spacer layer therebetween for imposing both a demagnetizing field and a sense current field on the free layer that is in an opposite direction to the first direction so as to counterbalance the pinned layer demagnetizing field and the sense current fields from the pinning layer, the pinned layer and the first spacer layer.
It is desirable that the keeper layer have a high resistance so that the amount of sense current shunted is small. The keeper layer should also have high magnetic moment to minimize its thickness to reduce current shunting and to fit in the read gap. It is further desirable that the keeper layer have soft magnetic properties (low intrinsic anisotropy) so that when the sense current is conducted through the sensor its magnetic moment will be perpendicular to the ABS in a quiescent state (no field signal from the rotating magnetic disk). Shunted sense current raises the temperature of the sensor which can contribute to instability of the pinning layer and protrusion of alumina layers in the magnetic head. The intrinsic anisotropy (H.sub.K) is the amount of applied field required to rotate the magnetic moment of the keeper layer from its easy axis (no applied field) to a position 90.degree. to the easy axis. Low intrinsic anisotropy is desirable so that the magnetic moment of the keeper layer can be easily rotated to the desired direction perpendicular to the ABS should the easy axis of the keeper layer be nonperpendicular to the ABS. Even though the easy axis of the keeper layer may be constructed perpendicular to the ABS an asperity on a rotating magnetic disk or fields from electrical pulses through the sensor, such as electrostatic discharge (ESD) or electrical pickup during manufacture or in the disk drive, can heat the sensor to a temperature in the presence of magnetic field that causes the easy axis to be reoriented nonperpendicular to the ABS. If the keeper layer has a high intrinsic anisotropy the sense current fields from the other conducting layers may not be sufficient to rotate the magnetic moment of the keeper layer to the desired direction perpendicular to the ABS. With high keeper H.sub.K the keeper magnetization can get stuck perpendicular to the ABS in a direction reversed to the desired direction. When the magnetic moment of the keeper layer is not perpendicular to the ABS a component of its demagnetizing field is parallel to the ABS which reduces the counterbalancing effect of the keeper layer.